The present invention relates to a picture image compression system in which, in coding for the purpose of narrow band transmission or storage of picture images, a variable length code is adaptively provided, depending on the combination of respective sequences converted into the spatial frequency domain, through Hadamard transformation.
It is well know that it is possible to achieve a reduction in transmission requirements or to bandwidth compress signals by deleting redundant signals in view of the statistical characteristic of a picture image or by deleting signals unnecessary for an acceptable visual characteristic.
High efficiency coding can be achieved if a bias can be applied to a picture image in any picture image transformation by using the fact that picture image signals have a strong spatial correlation between adjoining picture elements. A linear prediction transformation system and an orthogonal transformation system are well known as a transformation system for providing a bias to a picture image. The linear prediction transformation system utilizes the fact that when adjoining picture elements are predicted, the prediction error has a statistical deviation, while the orthogonal transformation system utilizes the fact that when a picture image is transformed into the spatial frequency domain, there is a deviation in the spectra of the respective frequency components which are the output of the transformation. In order to perform effective coding, it is preferable to allow distortion to a visually allowable degree or to allow quantization distortion in these transformation components in order to reduce the amount of transmitted information. Furthermore, since the entropy becomes small when there is a deviation in the probability distribution of these transformation components, it is preferable to give a variable length code adaptive to this effect. The statistical deviation varies considerably depending on the picture image and has such a characteristic that the deviation is large in the case of a smooth picture image, while the dispersion is large in the case of a complex picture image. As a result in variable length coding, the compressibility or the amount of information in a compressed picture image varies widely depending on the picture image. This means that when the input of picture image as well as the output rate are constant, the application of variable length code to the picture image produces a variation in time base with respect to the amount of transmitted information. Accordingly, it is required to correct the variations in time base.